Some Internet Service Providers (ISPs) may block the normal sendmail port 25, so the easiest way is to contact them and ask for the Outgoing Server SMTP setting which you will then replace (Outgoing Server: mail.yourdomain.com) with above.

Alternatively if your ISP is blocking port 25 you can try port 465 or 587 instead of 25.

Here is a simple test you can find out if this is the case.

Please note that on newer versions of Windows, Telnet is not enabled automatically. You can turn on Telnet in your computer's Control Panel by clicking on Programs and Features, then clicking on Turn Windows features on or off and ticking the box nex to Telnet.

Windows users can access this via start > run > cmd. Mac users can use the Terminal application.